Melanoma is a usually fatal skin cancer, unless it is detected and surgically removed in its earliest stages. Early detection of malignant melanoma is difficult because early melanomas, those having a Breslow thickness less than 1 mm, share many diagnostic features with benign lesions, such as dysplastic nevi or atypical melanocytic nevi.
To aid in the analysis of lesions, conventional photography, referred to as "clinical imaging", has been used to image the lesion for further study. The effectiveness of clinical imaging can be compromised, however, by specular reflection by the skin. Polarizers have been used for polarized imaging, which minimizes specular reflection.
Dermoscopy is another technique for examining skin, in which specular reflection is minimized. Dermoscopy also assists in clinically differentiating melanoma from its benign simulants by enabling the observation of features of pigmented melanocytic lesions that are not discernible by the naked eye. In dermoscopy, the skin is made more transparent to light by providing an oil layer over the skin, in front of the optical system. A glass plate is placed over the oil layer. The oil has an index of refraction between the index of refraction of the horny layer of the skin and the glass plate. Standard magnifying optics may be used to enlarge the structures rendered visible on and under the surface of the skin by the oil layer. The region of interest can then be examined visually. Slides of the region of interest can be made, as well, for future study.
Despite their similarities, most malignant melanomas differ in certain of their characteristics from other melanocytic lesions. A major advance in characterizing skin lesions based on certain of the observable differences between malignant and other lesions is the "ABCD" rule, where A=asymmetry, B=border irregularity, C=color variability, and D=diameter greater than 6 mm. A corresponding ABCD rule, where "D" refers to dermoscopic structures, such as brown globules, black dots or pigment networks within the lesion, is applied to dermoscopic images. Because the clinical and dermoscopic applications of these rules are subjective, they are not very reliable.
When skin is illuminated by light, the light can be re-emitted by reflection, scattering or fluorescence. It is known in the art that re-emission of light absorbed at different wavelengths by the region of interest can provide different information. For example, as the wavelength of the light increases, its depth of penetration into the skin also increases. Chromophores at different depths in the tissue therefore absorb and re-emit light at various wavelengths. Melanin and hemoglobin are examples of such chromophores.
Since the unaided eye cannot perceive light outside of the visible region or low-contrast structure in visible-light images, information which may be useful in diagnosing a lesion may not be directly observable. Digital acquisition and processing of dermoscopic images may, therefore, improve diagnostic reliability by employing more of the information residing in such images that is not directly observable. There have therefore been attempts to use objective, computer-based, image analysis algorithms that can discern meaningful differences between benign and malignant melanocytic lesions with sufficient accuracy.
Computer processing of images requires that the image be in digital form. A digital image is an array of digital signals whose values are a function of certain characteristics of the subject of the image. When imaging skin lesions, the digital images comprise digital signals whose values are a function of the re-emission characteristics of the skin and lesion, at different spectral bands of the light. The array is obtained by spatial sampling and quantizing the intensity of images obtained with film or directly by electronic cameras. Practical limitations on the number of picture elements or pixels per unit area of image determine the achievable spatial resolution of the digital image. The digital image typically needs to be segmented to separate the digital signals which are a function of the skin lesion from the digital signals which are a function of the surrounding skin.
Computer aided analysis has also been used to classify skin lesions using quantitative values indicative of particular characteristics of lesions, referred to as parameters. Based on histopathological diagnosis of lesions, algorithms have been developed which use linear or non-linear classifiers to combine parameters provided by the operator of an imaging device or a physician or computed by a processor, to yield a value which can be used to classify the lesion. Because some of the steps in the computer-aided analysis of which we are aware depend on subjective judgments of an individual, such analysis may provide highly variable results.
The images heretofore available have been obtained with commercially available red-green-blue color imaging apparatus. Color photographic transparencies of skin lesions have been digitized and skin lesions have been directly imaged with "three-chip" digitizing cameras. Such cameras employ broad-band filter bandpasses that are ultimately based on the wavelength response of the human visual system and have large regions of overlap.
Electronics images may also be obtained in narrower, non-overlapping filter bandpasses, which may reveal additional, wavelength-dependent differences between the images of melanomas and of benign lesions. However, such devices have had poor resolution and/or poor signal-to-noise characteristics which prevent the acquisition of digital images of melanocytic skin lesions of sufficient quality for effective application of machine vision techniques for lesion diagnosis.
Existing imaging systems and processes also tend to suffer from an inability to provide the required repeatability of the value of extracted lesion parameters, due in part to a lack of standardization with respect to spatially varying artifacts, so that the parameters, therefore, lack invariance to lighting and image exposure conditions, for example. Obtaining high signal-to-noise ratios in images recorded in narrow filter bandpasses, when exposure times are sufficiently short that the skin is effectively "frozen" during the exposure sequence, has also been difficult. In addition, since the optimum wavelengths for automatic characterization may not be the optimum wavelengths for visual observation, it may be difficult to reconstruct high-fidelity color images from the digital images for visual interpretation by a clinician.
The assessment of wounds and burns through the appearance of color images present similar challenges. Existing technology for the imaging of skin in vivo for these purposes is also inadequate. Practical solutions to the problems of employing multispectral digital imaging of skin for the analysis of lesions, wounds, or other conditions, have not been found.
The methods and systems of the present invention provide for the acquisition of digital images of skin at a plurality of spectral bands to automatically characterize the condition of the tissue based on the digital images. Spectral wavelength bands within and outside of the visible band may be used. In accordance with the present invention, a pigmented skin lesion can be characterized as malignant or benign, for example. Wounds or burns can also be characterized with respect to their rate of healing. The digital images comprise a plurality of digital signals whose values are functions of the condition of the tissue. The digital images acquired are subjected to objective and quantitative analysis by a digital processor to detect and identify abnormalities. The analysis includes image segmentation, parameter estimation and characterization of the skin. The estimation and characterization steps are automatic. The segmentation step may be automatic, as well. Subjective judgments are therefore minimized or eliminated.
It has further been found that generating the segmentation mask from a digital image acquired with light in a spectral band which does not penetrate deeply into the skin, such as a spectral band with a center less than about 500 nanometers, provides superior results. After segmentation, estimated values which are functions of characteristics of the lesion, such as its texture, asymmetry, blotchiness, and border irregularities, are computed and used to automatically characterize the condition of the skin. Clinically significant dimensional parameters such as diameter may also be evaluated. Digital signals corresponding to hair or blob-like structures are preferably removed during segmentation. Some or all of the values may be estimated through wavelet maxima representations, as well.
In accordance with the present invention, a method for characterizing the condition of a region of interest of the skin, wherein the absorption and scattering of light in different spectral bands by the region of interest is a function of the condition of the skin, is disclosed. The method comprises illuminating the region of interest of the skin by light in at least three spectral bands and digitally imaging the region of interest at the at least three spectral bands with the light re-emitted by the skin to generate digital images comprising digital signals whose values are a function of the condition of the skin. The digital images are provided to a processor which segments the digital images by generating a segmentation mask from a digital image in any one of the at least three spectral bands. The processor estimates at least one rotationally and translationally invariant statistical measure of coefficient distributions of the multiscale wavelet maxima representations of the digital images in each spectral band, which are functions of the texture of the region of interest determined by the segmentation mask. The processor characterizes the condition of the skin based on the estimated values, and outputs the characterization of the condition of the skin. Preferably, the segmenting, estimating and characterizing steps are conducted without the intervention of an operator.
Additional parameters include measures of the texture, asymmetry, blotchiness and border irregularity of the portion of the region of interest.
The digital images may be obtained by directly imaging the region of interest with a digital camera, or digitally imaging color slides of the region of interest, through appropriately filtered light.
The characterizing step may include comparing a weighted combination of the parameter values against a threshold value. The weight coefficients for each parameter value and the threshold value may be selected based on a training set of images of lesions or other skin conditions, whose condition has been determined, preferably through histological examination by a plurality of doctors. Preferably, for skin lesions, specificity is maximized under the constraint of 100% sensitivity to melanoma.
In accordance with another aspect of the invention, a system for characterizing the condition of a region of interest of skin includes means for illuminating the region of interest with light in at least three spectral bands and a camera for acquiring digital images of the region of interest based on the light re-emitted from the illuminated region of interest at each of the spectral bands. The digital image comprises digital signals whose values are a function of the condition of the region of interest. A digital processor segments the digital images by generating a segmentation mask from a digital image in any one of the at least three spectral bands. The processor estimates at least one rotationally and translationally invariant statistical measure of coefficient distributions of the multiscale wavelet maxima representations of the digital images in each spectral band, which are functions of the texture of the region of interest determined by the segmentation mask. The processor characterizes the skin condition based on the estimated value or values. The other parameters discussed above may be used, as well.
The camera may be a single-chip or multiple-chip charge-coupled device which detects light in a plurality of spectral bands between the near ultraviolet to near infrared. The filter means may be a plurality of interference filters mounted on a wheel for stepping any filter into a position intercepting the light from the light source. Preferably, at least one of the spectral bands has a center which lies between about 350 and 500 nanometers, at least one of the spectral bands has a center which lies between about 500-600 nanometers, and at least one other spectral band has a center which lies between about 750-1000 nanometers.